1. Field of the Invention
This invention relates to a method for controlling a shape of a plate, which method controls a rolling shape of a plate by a rolling mill having a roll bending mechanism.
2. Description of the Prior Art
Generally, in the rolling mill equipment for rolling a steel plate, for example, such as a thick plate, various attempts have been made in order to finish steel plates sequentially delivered with a uniform thickness or a good surface precision in both longitudinal and width directions. Among them, for the control of a longitudinal shape of a plate, rolling control is employed which is based on a wired logic incorporating a feedback control, a sequence control or the like in a rolling line, and an Automatic Gage Control (AGC) device or the like relying on a reference thickness of a plate has been used. However, the control of a widthwise plate thickness of a plate cannot be carried out by the AGC device. In view of this, a roll bending method is used as a method for controlling a width thickness of a plate. Accordingly, a roll bending apparatus applied to this method intentionally bends a roll externally by means of hydraulic pressure or the like to thereby effect accurate control when the roll is put in a stand.
FIGS. 1A and 1B are a perspective view and a fragmentary sectional view, respectively, showing a shape or form of a plate rolled by a conventional rolling mill. In these figures, reference numeral 1 designates a plate. Where the coefficient of extension of the plate 1 is not even in the width direction of the plate, undulations 2 are produced at the edges thereof. When the plate is undulated upwardly through m with respect to a fixed length l of a flat portion 3, the undulation extends as indicated by l', and a steepness .lambda. of said undulation is expressed by m/l. The following equation is obtained. EQU (l'-l)/l=2.5.lambda..sup.2
FIG. 2 is a conceptual view of a plate crown. A work roll 4 in contact with the plate 1 and a backup roll 5 adjacent thereto are subjected to reaction of the plate 1 to produce a bending phenomenon, thus producing crowns as indicated by flexed curves 4a and 5a.
In FIG. 2, the hatched portion a indicates an approximate value of a contact elastic strain amount of both the rolls 4 and 5, and the hatched portion b indicates a contact elastic strain amount of the work roll.
On the other hand, a shape control method for presetting a roll bending force in order to obtain an intended plate shape is disclosed, for example, in Japanese Patent Publication No. 15253/1977 publication. According to this publication, a calculation formula of a roll bending force for making constant the coefficient of extension widthwise for determining a plate shape is theoretically studied to provide a relative formula, by which the bending force is determined.
In the conventional method for controlling a shape of a plate, the roll bending force for making constant the coefficient of extension widthwise is derived from the calculation formula. However, it is not assured that a value obtained therefrom positively falls into a hardware of a rolling mill in terms of restriction of the hardware. Therefore, a problem occurs that a pattern is not always realizable.
In addition, even at present, iron manufacturers are working hard to elucidate the calculation formula but cannot obtain it easily.